Copper foil is used in the making of printed circuits. Electro-deposited copper foil, the kind of foil most commonly used, is made by depositing copper on a cylinder rotating in a copper plating bath and then stripping the layer of deposited copper foil from the cylinder. The surface of the foil adjacent to and in contact with the cylinder is a replica of the cylinder surface. Since the cylinder surface is usually polished, that copper foil surface is "shiny" and is so designated, distinguishing it from the other side, the matte side, of the copper foil. The matte side is generally crystalline with micro peaks of several microns or greater in height. The surface character of the matte side is determined by the conditions of electro-deposition - bath composition, current density, temperature, thickness of deposit, addition agents and the like.
Electro-deposited copper foil is described, for example, in Sarazin et al., U.S. Pat. No. 3,984,598 (1976); Luce et al., U.S. Pat. No. 3,585,010 (1971); and in Rider et al., "Printed and Integrated Circuitry," (New York: McGraw-Hill 1963), pp. 24-27.
Sarazin describes known deposition methods, including one where a thin metal-clad laminate is prepared by depositing a copper coating on a substrate treated with a release agent, treating the exposed copper surface to improve adhesion, bonding the treated surface to a resinous laminate using a bonding agent, and removing the substrate, which is weakly bonded to the copper because of the release agent. The copper surface is treated by subjecting the copper to a high current density, to cause roughening, and oxidizing the surface by adding heat, to prime the surface for the bonding agent, such as a silane compound. Luce describes conventional electro-deposition methods which incorporate a protective barrier layer between the copper layer and the resinous laminate. Rider provides an overview of known copper deposition procedures.
When the copper foil is bonded or laminated to a resinous substrate such as epoxy, polyimide, phenolic and the like, which may be reinforced with glass or other fibers, good adhesion between the copper layer and the resinous substrate is important to prevent separation in subsequent procedures such as soldering, or from thermal or mechanical stress. Accordingly, the matte side of the copper foil, rather than the shiny side, is used for lamination since the surface roughness of the matte provides better adhesion. But, the matte side along usually does not have sufficient "roughness" for good adhesion and accordingly, the matte surface is treated to improve its adhesive strength.
The most commonly used treatment in commercial use today involves electrolytic deposition upon the matte side of a layer of copper microdendrites which may be further processed by known methods. This treatment, called "Treatment A" was invented by the present inventor in 1957 (but was not patented), and employs a high-current density short of burning, to deposit adherent microdendrites on the surface of the previously deposited copper foil. One such method, which builds on the original Treatment A, is disclosed in Luce,'010, supra, and Luce, U.S. Pat. No. 3,293,109. See also, Conley, U.S. Pat. No. 3,220,897.
A typical peel strength of a treated matte copper surface (copper thickness 35 microns) bonded to a glass-reinforced epoxy base, may be from 10-14 lbs/in. of width, whereas the peel strength of an untreated matte surface may be from 1-3 lbs/in. The peel strength of an untreated shiny side will be less than 1 lb/in. When applied to the shiny side of copper foil, an electrochemical treatment such as Treatment A may provide a peel strength of 7-9 lb/in. when bonded to an epoxy substrate. Although treatment of the shiny side alone by electrochemical means is not ordinarily done, sometimes both sides of the copper foil are treated. However, this two-sided treatment presents difficulties in processing and handling.
In subsequent steps for making printed circuits, the untreated shiny side of the copper clad laminate is coated with a resist in selected areas by photomechanical or other procedures and the uncoated copper areas etched away down to the substrate of the laminate. When the resist is then removed, a printed circuit or "printed wiring" is produced. When appropriate components such as resistors, transistors and the like are attached to the copper clad laminate, for example by soldering, the result is a printed circuit board.
For reasons of economy and also of miniaturization, two-sided printed circuit boards are frequently used. The circuits on the two sides are interconnected by holes which are drilled and then metallized by techniques known to the art. In two-sided boards, both sides of the board have treated matte surfaces of copper foil bonded to the resinous substrate.
A subsequent stage in the evolution of modern printed circuit board technology was the development of multi-layer boards which may comprise from four to as many as twenty or more circuit sides. Such multi-layer structures require that the shiny side of the copper foil after etching to make circuits, be bonded to resinous surfaces in the interior circuit layers. The resinous surfaces may comprise those of prepreg layers, that is, B-stage glass-reinforced resinous layers which under heat and pressure act to bond together adjoining copper circuit sides. Since adhesion to the adjacent resinous substrate of the shiny side surfaces of the copper circuit lines would be very poor otherwise, these shiny side copper circuit lines must be treated to provide adequate bond strength so as to prevent delamination from thermal and other stresses.
The copper circuit lines on these boards are usually disconnected elements, that is, not in electrical continuity one to another. Therefore, further electrolytic processing to improve adhesion, such as that used in the Treatment A process above mentioned, is not feasible. Chemical processes must be used instead. The most frequently used procedure in commercial practice at the present time, called "black oxide" or "brown oxide" processing comprises oxidation of the shiny side copper surface in alkaline solutions at elevated temperatures by oxidizing agents such as sodium chloride. As a result of such black or brown oxide processing, peaks or whiskers of copper oxides (cupric/cuprous) are produced. The increase in surface area and also the locking-in effect of the whisker microtopography of the surface to the resinous substrate results in improved adhesion, with peel strength values of from 5-7 lbs/in. or so. Black oxide processing is described for decorative purposes, for example, in two patents to Meyer, U.S. Pat. Nos. 2,364,993 and 2,460,896, and one patent to MacMahon, U.S. Pat. No. 2,481,854. Black oxide treatments for bonding copper foil to laminates for printed circuit purposes are described in Rider, supra., p. 27.
Copper oxides such as cupric/cuprous oxides are soluble in acid, and serious difficulties may arise as a result of later process steps which involve the use of acid. For example, while drilling holes for through connections, epoxy resin is smeared on the surface of the holes. To remove the smear and also to remove glass fiber fragments, acids such as sulfuric acid and hydrofluoric acid are frequently used. These acids can attack the copper oxide whiskers at the interface or junction between the copper surface and the adjacent resinous layer, resulting in the so-called "pink ring" phenomenon. Pink ring consists of a copper surface from which the black or brown copper oxide has been dissolved away. Such pink rings may extend several millimeters or more inward from the hole circumference. The bond between the circuit area and the resinous layer is substantially destroyed and this may lead to serious delamination problems. The present invention is especially useful in obtaining good through-hole connections with improved bonding, superior mechanical and electrical properties, and without "pink ring."